Plain coffee is gluten-free unless it has been contaminated from another source. It is possible for coffee to become contaminated with gluten during the packaging process, or while being prepared at home or a restaurant.
Gluten comes from wheat, rye and barley. Some flavored coffees contain barley-based natural flavors or flavors in a grain-alcohol base. These substances can trigger gluten-sensitivities. In other cases, plain coffee causes problems in people with gluten sensitivities or celiac disease because coffee in general is hard on the digestive system not because of gluten. Caffeine in particular causes gastrointestinal problems in some people. Cutting back on the amount of caffeine ingested and trying different brands of coffee sometimes helps the problem.